Digital display technology has become one of the largest branches in the field of modern electronics and optoelectronics and generates demands in various applications where an image forming function is needed. Among those applications, projection-based display, which has the potential of generating a large size image with low cost, is of great interest.
In a conventional projection system based on a passive imager device, such as liquid crystal display (LCD), digital mirror devices (DMD), and liquid crystal on silicon (LCOS), the passive imager device itself does not emit light. Specifically, the conventional projection system projects images by optically modulating collimated light emitted from a light source, i.e., by either transmitting, e.g., by an LCD panel, or reflecting, e.g., by a DMD panel, part of the light at the pixel level. However, the part of the light that is not transmitted or reflected is lost, which reduces the efficiency of the projection system. Furthermore, to provide the collimated light, complex illumination optics is required to collect divergent light emitted from the light source. The illumination optics not only causes the system to be bulky but also introduces additional optical loss into the system, which further impacts the performance of the system. In a conventional projection system, typically less than 10% of the illumination light generated by the light source is used to form the projection image.
Light-emitting diodes (LEDs) made of semiconductor materials can be used in mono-color or full-color displays. In current displays that employs LEDs, the LEDs are usually used as the light source to provide the light to be optically modulated by, e.g., the LCD or the DMD panel. That is, the light emitted by the LEDs does not form images by itself. LED displays using LED panels including a plurality of LED dies as the imager devices have also been studied. In such an LED display, the LED panel is a self-emissive imager device, where each pixel can include one LED die (mono-color display) or a plurality of LED dies each of which represents one of primary colors (full-color display). However, the light emitted by the LED dies is generated from spontaneous emission and is thus not directional, resulting in a large divergence angle. The large divergence angle can cause various problems in an LED display. For example, due to the large divergence angle, the light emitted by the LED dies can be more easily scattered and/or reflected in the LED display. The scattered/reflected light can illuminate other pixels, resulting in light crosstalk between pixels, loss of sharpness, and loss of contrast.